Natural and synthetic leather are used to make many useful products such as clothing, shoes, luggage, automobile interiors, upholstery, wallets, cases for eyeglasses and other things, handbags and belts, to name but a few. It is beneficial to treat leather to protect it from scuffing, scratching, flexing, humidity and cracking. In addition to protection from the aforesaid damages, it is also desirable for such coatings to provide a continuous film or coating having good visual and aesthetic properties such as surface smoothness (also known as “hand” or “handle”), non-tacky surface, homogenous color and texture, and appropriate gloss level.
Various methods of treating leather are known. Typically, such treatments consist of applying a coating to the outer or upper surface of the leather substrate with one or more compositions which are selected depending upon the specific characteristics required for the intended use of the leather. There are also multiple acceptable techniques for applying such compositions to leather substrates, including brushing, rolling, spraying, swabbing, flow coating and laminating. The application technique is, of course, selected depending on the nature of the coating composition and the type of leather product to be made of the coated leather.
Some leather coating compositions are made of casein, a protein derived from milk, that is cured with formaldehyde or glutaraldehyde, or even first mixed with alkoxy alkyl ureas and then cured with an acid catalyst (see, e.g., U.S. Pat. No. 4,374,872). Nitrocellulose lacquer is another widely used material for coating leather substrates, but requires pre-treatment with an adhesive layer to bind to the leather and typically results in a hard, non-tacky, glossy finish. U.S. Pat. No. 3,930,921 describes a two-layer coating for leather substrates having a top nitrocellulose lacquer layer and a sub-layer of cross-linked polyacrylate resin which eliminates the need for adhesive pretreatment. Polyurethane is also commonly used for providing a protective coating for leather substrates. For example, U.S. Pat. No. 5,061,517 discloses a brush-on composition for footwear and other leather products that comprises a polyurethane elastomer in toluene and isopropyl alcohol solvent mixture and which provides a thin, flexible, scuff-resistant coating. U.S. Pat. No. 5,872,182 describes one- and two-component water-based sulfonated polyurethane compositions for use as an adhesive, coating and primer for footwear and other leather products. Generally, from a health, environmental and safety standpoint, aqueous-based coating compositions are currently preferred over organic solvent-based compositions because they have reduced or even zero volatile organic compound (VOC) emissions.
Additional components, such as colorants, plasticizers, thickeners, gloss control agents and plasticizers, among others, are often added to leather coating compositions to modify certain characteristics otherwise imparted by the coating composition. For example, plasticizers are often added to lacquer compositions to make the lacquer coating more flexible, but this sometimes results in surface tackiness over time (see U.S. Pat. No. 5,061,517). U.S. Pat. No. 5,061,517 acknowledges the possibility that additional components may be included in leather coating compositions, for various purposes.
Depending on the product or use to be made of the treated leather, high gloss or low gloss may be preferred. When gloss control is employed, it remains important to control or maintain other desirable characteristics such as flexibility, scratch resistance and impact resistance.
One way to lower gloss has been to add matting agents, which are either inorganic particles, such as, for example, silica, or calcium carbonate particles, or organic particles, such as, for example, polyurethane dispersions, that function by roughening the surface of the film, or water dispersed acrylic acid dispersants which function to coagulate and disrupt film formation. For example, U.S. Pat. No. 5,002,584 describes a seasoning (top) coating for leather substrates which comprises cellulose acetobutylate or polyether siloxane, and to which has been added matting agents comprising silicone dioxide or aluminum oxide. U.S. Pat. No. 7,842,129 discloses a transparent lacquer formulation for coating wood, metals, plastics, and natural and synthetic leathers that comprises silicon dioxide matting agents. While effective at reducing gloss, such matting particles often disrupt film integrity and can lead to loss of key film properties such as water and abrasion resistance, slip, etc. Still further, the matting particles tend to polish or burnish after rubbing, leading to an increase in the gloss over time. Inorganic matting particles also tend then to settle out of solution.
Others have unsuccessfully attempted to overcome the problems associated with the use of matting particles by controlling of particle size, minimizing random light scattering, careful selection of refractive index between particle and matrix, or modifying the matrix or film forming polymer. However, such approaches sacrifice clarity or film properties for matte effect and can greatly limit the scope of useful compositions.
The present invention provides aqueous coating compositions having low gloss for application to leather substrates.